In the United States, women receive
conflicting advice about when to wean their children completely
from breastfeeding. The American Academy of Pediatrics recommends
one year, while WHO and UNICEF recommend at least two years. Many
physicians consider six months to be "extended"
breastfeeding, and some health professionals question the motives
of women who nurse for more than a year. In turn, women may hide
the fact that they are still nursing an older child from
disapproving health care professionals or family members. From
anthropological research, we know that in many non-Western
cultures children are routinely nursed for three to four years.
Are they eccentric, or are we? Can we look to other animals to
determine what the natural age at weaning would be in modern
humans if it was not modified by cultural beliefs?

Like all mammals, humans have mammary glands
that function to nurture their offspring. Within the class
Mammalia, humans are members of the order Primates, and have the
basic primate pattern of breastfeeding and weaning activity that
has been molded by more than 65 million years of natural selection
to ensure the best possible survival rate of primate offspring.
This basic pattern is assumed to be primarily genetically based.
In addition, a number of life-history variables are also
associated with age at weaning in the non-human primates. What do
these variables suggest about the "natural" age of
weaning in humans?

Weaning according to tripling or
quadrupling of birth weight

The idea that mammals wean their offspring
when they have tripled their birth weight is widely reported in
the breastfeeding literature (Lawrence 1989). This rule of thumb
holds true for small-bodied mammals, but not for larger ones.
Recent research has looked at age at weaning and at growth among
large mammals, including primates. The research shows that weaning
occurs some months after quadrupling of the birth weight, rather
than tripling (Lee, Majluf and Gordon 1991). When do U.S. infants
typically quadruple their birth weight? For males, the average age
is around 27 months, and for females, around 30 months.

Weaning according to attainment of
one-third adult weight

Other studies suggest that primates are like
other mammals in weaning each offspring when they reach about
one-third their adult weight (Charnov and Berrigan 1993). Humans
come in different sizes, but 4 to 7 years of nursing would be the
weaning age for humans using this method of comparison, with boys
generally being nursed longer than girls, and large-bodied
populations nursing longer than small-bodied groups.

Weaning according to adult body size

Harvey and Clutton-Brock (1985) published a
study of life-history variables in primates, including a formula
for calculating age at weaning based on adult female body weight.
The equation predicts an age at weaning for humans at between 2.8
and 3.7 years, depending on average adult female body weight, with
larger-bodied populations nursing the longest.

Weaning according to gestation length

It is often reported in the literature that,
among mammals in general, weaning age is approximately the same as
the length of gestation (Lawrence 1989). By this criterion,
weaning in humans might be expected to take place after only nine
months of breastfeeding. However, this one-to-one relationship is
greatly affected by the adult size of the animal. For many
small-bodied primates, the duration of breastfeeding is shorter
than the length of gestation. Among large-bodied primate species,
the duration of breastfeeding far exceeds the average length of
gestation. For humankind's closest relatives, the chimpanzee and
the gorilla, the duration of breastfeeding is more than six times
the length of gestation. Humans are among the largest of the
primates, and share more than 98 percent of their genetic material
with chimpanzees and gorillas. Based on these comparisons, an
estimated natural age at weaning for humans would be a minimum of
six times gestational length, or 4.5 years.

Weaning according to dental eruption

According to the research of Smith (1991),
many primates wean their offspring when they are erupting their
first permanent molars. First permanent molar eruption occurs
around 5.5 to 6.0 years in modern humans. It is interesting to
note that achievement of adult immune competence in humans also
occurs at approximately six years of age, suggesting that
throughout our recent evolutionary past, the active immunities
provided by breast milk were normally available to the child until
about this age (Fredrickson).

Our evolutionary past has produced an
organism that relies on breastfeeding to provide the context for
physical, cognitive, and emotional development. The human primate
data suggest that human children are designed to receive all of
the benefits of breast milk and breastfeeding for an absolute
minimum of two and a half years, and an apparent upper limit of
around seven years. Natural selection has favored those infants
with a strong, genetically coded blueprint that programs them to
expect nursing to continue for a number of years after birth and
results in the urge to suckle remaining strong for this entire
period. Many societies today are able to meet a child's
nutritional needs with modified adult foods after the age of three
or four years. Western, industrialized societies can compensate
for some (but not all) of the immunological benefits of
breastfeeding with antibiotics, vaccines and improved sanitation.
But the physical, cognitive, and emotional needs of the young
child persist. Health care professionals, parents, and the general
public should be made aware that somewhere between three and seven
years may be a reasonable and appropriate age of weaning for
humans, however uncommon it may be in the United States to nurse
an infant through toddlerhood and beyond.

References

Charnov, E. L. and D. Berrigan. Why do
female primates have such long lifespans and so few babies? or Life in
the slow lane. Evol Anthropol 1993; 1:191-94.